Phosphorus (P) is an essential nutrient for plant growth, development, and reproduction. After nitrogen, P is considered to be the second most important nutrient limiting agricultural production. Although the total amount of phosphorus in the soil may be high, it is often present in unavailable forms for plant uptake. For instance, Saskatchewan soils are inherently low in available P which ranges from 400 to 2000 lb/A in the top 6 inches of soil, but only an extremely small amount of the total P is available to the crop during a growing season. To reduce P deficiencies and ensure plant productivity, nearly 30 million tons of P fertilizers are applied to soils worldwide every year (e.g., P fertilizer is needed on about 85% of Saskatchewan cropland). Up to 80% of the P from the fertilizer is lost because it becomes immobile and unavailable for plant uptake because of adsorption, precipitation or conversion to organic forms. Moreover, in recent years increasing attention has been paid to the effect of excessive use of fertilizers, particularly P, in environmental pollution.
P helps plants store and use energy from photosynthesis to move nutrients into the plant and between cells, and produce sugars, starch and protein required for growth. In response to inorganic P limitations, some plants undergo physiological and developmental adaptations to scavenge limited phosphate from the environment, including decrease of their inorganic P consumption and mobilization of their inorganic P reserve. Phospholipids are the main form of cellular inorganic P reserve and their content markedly declines in plant during inorganic P starvation. In plants, glycerol-3-phosphate (G-3-P) is an obligated precursor for phospholipids synthesis, Further, G-3-P metabolism reflects perturbations of the general metabolic network.
The G-3-P synthesis enzyme, NAD+-dependent glycerol-3-phosphate dehydrogenase, is possibly a metabolic link between the cytosol and mitochondria that is necessary and essential for cellular redox control. Thus, G-3-P metabolism, particularly the activity of cytosolic G-3-P dehydrogenase is critical for many aspects of plant stress tolerance, including inorganic P limitation adaptation.
Glycerol-3-phosphate dehydrogenase (GPDH) (EC 1.1.1.8) is an essential enzyme for both prokaryotic and eukaryotic organisms. GPDH catalyses the reduction of dihydroxyacetone phosphate (DHAP) to glycerol-3-phosphate (G-3-P) using NADH as a reducing equivalent. Plant cells possess at least two isoforms of GPDH, one located in the plastids and the other in the cytosol.1 The purification of the cytosolic GPDH from spinach has been reported.2 The product of the reaction catalyzed by GPDH, G-3-P, is a precursor for the synthesis of all glycerol lipid species, including membrane and storage lipids. The biosynthetic role of this enzyme in bacteria was established in vivo by the isolation of glycerol and G-3-P auxotrophs of E. coli mutant strains deficient in its activity.3 These mutants could not synthesize phospholipid in the absence of supplemental G-3-P.
There are no reports of plant mutants defective in GPDH activity.
In addition to being essential for lipid biosynthesis, GPDH is involved in several other important biological processes. Most notably, GPDH, through consuming NADH and regenerating NAD+, plays an important role in maintaining cellular redox status. The NAD+/NADH couple plays a vital role as a reservoir and carrier of reducing equivalents in cellular redox reactions. For catabolic reactions to proceed, the ratio NAD+/NADH should be high. Under normal aerobic conditions, excessive NADH is channeled into mitochondria and consumed through respiration. Under anaerobic conditions, GPDH reactions serve as a redox valve to dispose of extra-reducing power. In this way, the cellular NAD+/NADH ratio can be maintained at a level allowing catabolic processes to proceed. The expression of the GPDH gene is subject to redox control and induced by anoxic conditions in Saccaromyces cerevisae. Deletion of the GPD2 gene (one of the two isoforms of GPDH) results in defective growth under anaerobic conditions.4 
GPDH has also been shown to play an important role in adaptation to osmotic stress in S. cerevisae. GPDH exerts its role in osmotic and salinity stress response through its function in glycerol synthesis. Glycerol is a known osmo-protectant. It is produced from G-3-P through dephosphorylation by a specific glycerol 3-phosphatase. To respond to a high external osmotic environment, yeast cells accumulate glycerol to compensate for differences between extracellular and intracellular water potentials.5 The expression of the GPDH gene, GPD1, has been demonstrated to be osmoresponsive.6 A strain of S. cerevisae in which the GPD1 gene has been deleted is hypersensitive to NaCl.7 Accumulation of glycerol as an osmoregulatory solute has been reported in some halophilic green algae including Dunaliella, Zooxanthellae, Asteromonas and Chlamydonas reinhardtii.8 
The sequence of a cDNA encoding GPDH activity has been reported for the plant Cuphea lanceolata.9 The encoded protein was tentatively assigned as a cytosolic isoform.
To date, there has been no report on the genetic manipulation of plant GPDH.